In general, a magnetic recording medium, such as a magnetic tape, is produced by coating and drying a magnetic material coating liquid comprising magnetic powder and a binder resin on a support.
In recent years, in a magnetic recording medium, particularly for a shortwavelength video recording, there has been a strong tendency toward the use of a more finely pulverized and more highly magnetized magnetic powder than ever before. Further, it has been prevailing to subject the surface of a magnetic layer or base film to smoothing treatment in order to prevent a spacing loss. The treatment, however, has resulted in increasing the coefficient of friction and deteriorating the runability of the medium to cause trouble such as tape adherence, edge crease, and stick slip. Particularly, the magnetic powder subjected to surface treatment is liable to result in a decreased conductivity and increased surface resistivity. Thus, the medium's runability is further deteriorated and dust is easily attracted to cause dropouts. There are known techniques to use carbon black for raising the individual characteristics of a magnetic recording medium, such as the conductivity, friction (runability), surface characteristics and abrasion resistance, as disclosed in Japanese Patent Examined Publication No. 15052/1981 for the conductivity, Japanese Patent Examined Publication No. 20203/1978 for the abrasion resistance (runability) and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese patent O.P.I. Publication) No. 51025/1981 for the surface characteristic (electromagnetic convbersion property). Also, there are known other techniques to use carbon black for improving both conductivity and friction characteristics of a magnetic layer, as disclosed in Japanese Patent O.P.I. Publication Nos. 5426/1984 and 16141/1984 and Japanese patent Examined Publication Nos. 20203/I978 and 9041/1979. However, where the conductivity of the magnetic layer is raised and the friction is reduced by these methods, the carbon black content of the layer is increased, which results in inferior dispersibility and stability of a magnetic coating liquid and deteriorated electromagnetic conversion properties due to a relatively reduced magnetic powder content. Furthermore, the adherence between a non-magnetic support and a magnetic layer is also deteriorated, and the binding amount of the binder in the magnetic layer is liable to be insufficient. The above matter tends to increase clogging of a magnetic head, which is caused by peeling of the magnetic layer from the support and dropping of the magnetic powder from the magnetic layer.
On the other hand, there have been proposed mediums which have a plurality of magnetic layers in order to improve and balance the magnetic recording characteristics both in high-frequency and low-frequency regions, as described in Japanese Patent O.P.I. Publication NOs. 172142/1984 and 64901/1976 and Japanese Patent Examined Publication Nos. 2218/1957 and 12937/1981.